The effective separation of undesirable substances comprises an important step in brewing beer. Mechanical processes, such as centrifugation and filtration have the advantage against thermal processes, for example heating for short periods, and conservation agents, that the products are neither thermally nor chemically burdened.
The quality of the product and/or its purity as well as the production costs play dominating parts in the brewing of beer. It is therefore required of the filtration installations which are used in the brewing process that they remove the accumulating turbid substances and yeasts and the microorganisms which are harmful to the beer, and simultaneously they do not adversely affect the aroma, taste and foreign body neutrality of the beer, while operating economically. A maximal filtration effect with small filter surfaces, high filtration speeds and long filter service life are expected. The filtration processes which are most used at present in the breweries are cake filtration with diatomaceous earth as the filtration ancillary agent and layer filtration using fiber and filter layers which contain diatomaceous earth. The separation of the particles is based on a screening effect and on the adsorption effect of the filter material. The great consumption of diatomaceous earth and the lack of possibility of regeneration of this filtration ancillary agent leads to great strains on the environment, so that alternative methods are being sought for a less problematical clarification of the beer. Great progress could be achieved with the development of membrane filtration, in which thin and highly porous membranes having a defined pore size, composed of plastic or ceramic materials, are used as the screen. Preference is given to the use of polymer materials of cellulose mixed esters, polyamides, polyimides, polyurethanes, polysulfons and nylon 66, as well as polyolefins, in particular PTFE. The pore sizes and the pore size distribution can be deliberately controlled by the manufacturing process so that the membranes are adjusted to predetermined particle size, wherein the maximal pore size should not exceed the size of the particles which are to be separated. In contrast to the diatomaceous earth and sheet filters, by suitable selection of the filter membranes, a predetermined particle size can be separated (detained) with defined certainty irrespective of the flow speed, of fluctuations of the throughput and of pressure surges. Further advantages of the membrane filters are that they do not adversely influence the filtered beer by emitting particles or by changing the contents. The membrane filters can be easily checked for their modus operandi, are easily handled, have an unproblematical endurance behaviour and a low maintenance expenditure. The membranes are produced either as highly porous films or in capillary and/or tubular form and can be applied on a support. A distinction is made between flat membranes (disc filters, spiral wound cartridge or pleated cartridge, plate and frame filters, dynamic pressure filters) and tubular or capillary membranes, while the membranes can be arranged in the corresponding modular form.
The membrane process can be classified in accordance with the size of the particles to be separated. As regards the permeability of the respective filter membrane, a differentiation is made between reverse osmosis, ultra-filtration, micro-filtration and conventional filtration, while the borders between the individual filtration stages overlap:
__________________________________________________________________________ SEPARATION LIMITS MG of the separable Size of separable Separable substances [g/mol] substances [.mu.m] substances __________________________________________________________________________ Reverse Osmosis .ltoreq.10 000 5.10.sup.-4 -5.10.sup.2 molecules, ions Ultrafiltration 10,000-300,000 5.10.sup.-3 -0,5 macromolecules (proteins, polysaccharides), kolloids viruses Microfiltration 100,000-1,000,000 5.10.sup.-2 -50 microorganisms, bac- teria, colloids, fine particles Conventional Filtration .gtoreq.1,000,000 &gt;5 particles __________________________________________________________________________
The individual membranes are frequently different not only in their pore sizes, but also in the membrane structure. Thus a differentiation is made between symmetrical membranes, the pores of which pass through the membrane layer with the same width, and asymmetrical membranes, the pores of which expand from one side of the membrane to the opposite side of the membrane. If the open pore side of the membrane is facing the non-filtered portion the membrane has a higher capacity for impurities. If the open pore side of the membrane is facing the filtered portion, particles, which are smaller than the pore diameter can more easily pass through the membrane and are not retained in the interior of the membrane.
The filtration technologies can be subdivided into conventional dead-end filtration and into dynamic cross-flow filtration or dynamic pressure filtration. In the first case the filtration is performed in such a way that the solution to be filtered is applied under pressure on the filter and permeates the filter, while the retained particles remain on the filter surface or are arrested in the pores. The entire liquid to be filtered is pressed through the filter. Because when using this process a filter cake rapidly forms on the surface, which increases the flow resistance greatly and reduces the throughput, only small amounts or quantities which have already been largely clarified can be filtered. The dynamic filtration avoids this disadvantage in the process technology because the liquid to be filtered is not simply brought under pressure onto the filter surface but the major proportion is guided tangentially, i. e. parallel to the filter surface and perpendicularly to the flow of filtrate at great speed over the filter surface (cross-flow filtration) or because the filter surface is moved at greater speed (dynamic pressure filtration), and therefore respectively only a proportion of the liquid penetrates through the filter. The retained substances therefore are not stored on the surface of the filter, do not form any impermeable filter cake and do not block the pores of the filter, but are led away by the high flow speed along the surface of the filter and are concentrated in the non-filtered portion. Thereby a high filtrate quantity is achieved until the filter is clogged.
In the filtration processes in brewing, it is advantageous that germs which are harmful to the beer should be filtered out. This filtration is conventionally performed using a membrane in accordance with the dead-end filtration principle. The membrane filters which are used in practice having a pore size of 0.8 microns indeed possess a high throughput and high permeability at acceptable pressures and therefore function economically, but microbiological sterility with respect to the bacteria damaging to the beer cannot be guaranteed. If membranes are used with a smaller pore diameter (0.45 microns, 0.20 microns), the bacteria which are harmful to the beer are removed by the membranes, but an economical modus operandi and therefore low cost beer production is no longer possible, as a rule, because of the low throughput and the increased tendency to clogging.
In another important filtration process, the beer is freed on its way from the storage tank from yeast, turbid substances and bacteria. In the case of dead-end filtration as the filter agent for the separation of the chill haze which consists of protein agglomerates, diatomaceous earth is still used, which however has the disadvantages listed above of high consumption, lack of regeneration possibility and great environmental pollution. Filter materials such as ceramics, Al.sub.2 O.sub.3 or polypropylene were tested as the membranes in a dynamic filtration for the separation of yeast and turbid substances, but they could not as yet achieve any technical breakthrough because of the low throughput.
In European patent application EP-A2 0 228 072 filter membranes are disclosed of polyethers, polyimides, polyamides and polyether sulfons having a pore size in the range from 0.02 to 20 microns. These membranes are used for preference in the electronic and pharmaceutical industries. There is no discussion in more detail of the use of these filters for the microbiological filtration of beverages, and in particular indications of the special problems of beer filtration are lacking.